In order to measure an amount of various components contained in an exhaust gas of an internal combustion engine or fuel consumption, it is necessary to measure a flow rate of the exhaust gas in addition to a concentration of the above-mentioned components. Conventionally, as shown in the patent document 1, the flow rate of the exhaust gas is measured by mounting a flow meter of an ultrasonic type on a tube body connected in series to, for example, a tail pipe of a vehicle or an internal combustion engine.
At this time, a temperature sensor is provided and temperature of the exhaust gas is also measured at a time of measuring the flow rate. The reasons are that the temperature of the exhaust gas is necessary in case that a normal conversion is executed on the measured flow rate (the measured flow rate is converted into a volume flow rate in a normal state, in other words 0 degree Celsius, 101.325 kPas·abs) and that the fluid temperature is a parameter at a time of measuring the flow rate not only for the ultrasonic wave flow meter but also for the flow meters of various types.
The temperature sensor is ordinarily arranged to locate its sensor body near the center of the flow channel so as to make it possible to measure the temperature of the exhaust gas that flows in the center of the flow channel.
In case of measuring a fuel efficiency of the internal combustion engine or measuring a component of the exhaust gas, it is, so to speak, a common knowledge that the temperature near the center of the flow of the exhaust gas flow indicates the temperature of the exhaust gas, and the temperature is considered to be a parameter at a time of calculating the flow rate of the exhaust gas. In addition, no case is found that any particular problem occurs.
However, under a special test condition such as an oil mixing test of the diesel engine, it becomes clear that an error occurs in the measured flow rate by the above-mentioned flow rate measurement method. As a result of keen examination of the present claimed inventor, this inventor first found the cause for why the error occurs.
More specifically, the tube body on which the flow meter is mounted is heated to about 100 degree Celsius by a heater in order to prevent condensation of water or THC on an inner wall surface of the tube body. In the above-mentioned oil mixing test, a rotational number of the internal combustion engine is raised at once and the flow rate is continuously measured by flowing the exhaust gas whose temperature reaches a maximum temperature of 300 degree Celsius for a considerable long period of time. Under this special condition, since the temperature of the inner wall of the tube body becomes considerably low compared with the temperature of the exhaust gas for a while after the exhaust gas starts flowing because a circumferential part of the exhaust gas is cooled down by the tube body, a big difference is caused between the temperature of the exhaust gas flowing near the inner wall of the tube body and the temperature of the exhaust gas flowing near the center where the temperature is kept constant. As a result of this, in accordance with a conventional method wherein the temperature of the exhaust gas in the center is measured, the influence on the circumferential part of the exhaust gas due to temperature drop is ignored, resulting in a measurement error.
In contrast, in a general mode running test, there is no chance that the exhaust gas of the high temperature continues flowing at once at a first arrival time so that it is hardly possible there is some problem in measuring the flow rate by the conventional measurement method.